1. Field of the Invention
The present invention relates to a water-based ink set for ink-jet recording for reducing bleeding between a black ink (hereinafter referred to as a Bk ink) and a yellow ink (hereinafter referred to as a Y ink), and for reducing color unevenness in a portion formed by color mixing of color inks.
2. Description of the Related Art
In an ink-jet recording method, recording is performed by forming droplets of ink and causing all or a part of the formed ink droplets to adhere to a recording material such as paper. In this case, the ink droplets are formed by means of an ink ejection method such as an electrostatic attraction method, a method in which mechanical vibrations or displacements are applied to ink by use of a piezoelectric element, or a method utilizing the pressure generated when ink is heated to generate bubbles.
As an ink for ink-jet recording employed in such an ink-jet recording method, an ink has been known which is formed by dissolving or dispersing a dye or a pigment in a liquid medium formed of water and a water-soluble organic solvent.
An ink for ink-jet recording is required to have optimal performance for use in an ink-jet recording method. Specifically, for example, high print quality is required when an image is formed on recording paper. That is, the ink does not cause irregular flow along fibers of the recording paper and irregular penetration to the space between the fibers (hereinafter referred to as blurring), whereby the image has sharp edges. The ink is also required to dry rapidly and to have excellent rub fastness when an image is formed on recording paper. Further, the ink is required to be capable of being stably ejected for a long time. That is, the ink does not cause corrosion of a resin material or a metal material employed in an ink-jet head, whereby a nozzle is not clogged. In view of the above and other requirements, various improvements have been made. Moreover, in recent years, there has been an increasing demand that recording be made on ordinary paper instead of paper designed specifically for ink-jet recording in terms of cost and environmental friendliness.
However, when recording is made on ordinary paper by use of conventional inks, different colors are mixed in a boundary region in which the colors are adjacent to each other (hereinafter referred to as bleeding), resulting in a problem of deterioration in print quality. Between a Bk ink and a Y ink, this bleeding is highly noticeable since the lightness difference therebetween is large and thus is a particularly serious problem.
As a measure for solving such problems, a method has been known in which a compound such as a surfactant for enhancing penetrability is added to ink (Japanese Patent Application Laid-Open No. Sho 55-65269). According to this method, although bleeding between a Bk ink and a Y ink is improved, there is a drawback that color unevenness in a portion formed by color mixing deteriorates when color inks are employed. In order to improve the color unevenness in a portion formed by color mixing when color inks are employed, an alkyl ether of polyhydric alcohol, such as diethylene glycol butyl ether which is applied in a commercial ink for color ink-jet recording, may be added as a penetrant to the inks. However, in this case, if a large amount of an alkyl ether of polyhydric alcohol is added, bleeding between the Bk ink and the Y ink becomes serious. Therefore, in a conventional method, it has been very difficult to reduce both the bleeding between a Bk ink and a Y ink and the color unevenness in a portion formed by color mixing of color inks at the same time.